The present invention relates to high-temperature electrochemical cells that employ molten-salt electrolytes. More particularly it concerns a negative electrode composition which exhibits improved structural integrity on cycling.
A large amount of work has been done in the development of high-temperature electrochemical cells. Positive electrodes for use in such cells have included chalcogens such as sulfur, oxygen, selenium or tellurium as well as their transition metal chalcogenides. Positive electrode materials such as the sulfides of iron, cobalt, nickel and copper are commonly employed.
In high-temperature cells, current flow between electrodes often is transmitted by molten electrolytic salt. Particularly useful salts include compositions of the alkali metal halides and/or the alkaline earth metal halides ordinarily incorporating a salt of the negative electrode reactant metal.
Alkali metals such as lithium, sodium, potassium and alloys of these materials are contemplated as negative electrode reactants. Other cells have included the alkaline earth metals such as calcium, magnesium, etc. in negative electrodes. Alloys of these materials such as lithium-aluminum, lithium-silicon, lithium-magnesium, calcium-magnesium, calcium-aluminum, calcium-silicon, magnesium-aluminum have been investigated to maintain the negative electrode in solid form rather than as molten metal at the cell operating temperatures.
In prior lithium-aluminum electrodes the loss of lithium on electrical discharge has resulted in near complete degradation of the electrode structure. Aluminum without alloy additives appears unable to maintain a coherent electrode structure during electrochemical cycling. It crystallizes in patches and particles such that the original electrode form is lost.